This invention relates to a method and apparatus for producing a reinforced molding compound in a sheet form. More particularly the invention relates to an improved compaction section or wet out station to be used in forming the molding compound.
In the past a number of devices have been used to produce molding compounds in sheet form. Usually these devices deposit a reinforcing material and a resinous material molding compound in a pre-molded state between two carrier sheets. Then the composite retained as a layer between the carrier sheets, is passed between two opposed compaction rolls or a series of opposed compaction rolls that are in spaced apart relationship to wet out the reinforcement and resin. However, the prior art compaction rolls only supply force for a short period of time to the composite and the reinforcements are not always completely wet out by the resin. Also when the composite is formed there is frequently air entrapped beneath the carrier sheets. When the composite passes through the compaction rolls the air is not always forced from beneath these carrier sheets. After the composite passes through the compaction rolls the composite springs back and large air bubbles can be formed beneath the carrier sheets. These air bubbles can break the carrier sheets leading to production line shut-down or cause non-uniformities when the composite is molded. In addition the unequal force applied by the compaction rolls, the springing back of the composite after it passes through the compaction rolls and the air bubbles can cause the reinforcement in the composite to be displaced or moved. When the reinforcement is displaced the strength provided by the reinforcement may no longer be uniform as the positioning of the reinforcement in the composite is no longer uniform. Accordingly, the strength of an article molded from the composite may not be as uniform as desired.
A complete wet out of the fibers of the composite is essential to the quality of the ultimately molded product. The difficulties heretofore encountered in achieving proper wet out have resulted in limitations both in continuous process production rates as well as in the restriction of fiber loading or weight and thickness of the sheet molding compounds. When properly wet out, the entire surface of each fiber within the resin matrix is coated with resin. Failure to properly wet out a fiber of fibers generally results in the presence of an internally disposed air pocket or bubble representing a production defect.
In any event the non-uniformities that may be produced by the compaction rolls can reduce the uniformity of the sheet molding composite that is produced. This in turn can effect the uniformity of article molded from the composite. These non-uniformities can also effect the reproducibility or uniformity of composites that are produced at different times.
According to the invention there is provided a method and apparatus defining a means for linearly advancing a sheet molding compound. A moveable upper surface formed by a belt passing around a series of driven rollers is adapted to contact the upper surface of the sheet molding compound. The belt of the upper surface is held in biased relationship to the sheet molding compound.
An object of the invention is to provide an improved method and apparatus for making sheet molding compound.
Another object of the invention is to provide an improved method and apparatus for compacting and effecting wet out of the fibers of the sheet molding compound.
The invention is further addressed to an apparatus and method for treating sheet molding compound present as a composite of a molding compound in a premolded state having reinforcing fibers disposed therewithin, the composite being retained as a layer between the mutually inwardly disposed surfaces of two continuous thin, supporting sheets. Both the composite and supporting sheets are moved along a production path through two mutually facing continuous belts which preferably are steel mesh belts under tension. By arranging the roller supports of the belts appropriately, the composite and supporting sheets are caused to be not only compressed but to flex in vertically undulating fashion to derive a very high quality of wet out of the fibers within the resin matrix. In one arrangement, one belt is moved or driven at a fixed rate, while the belt adjacent thereto is driven at a rate continually oscillating a predetermined amount above and below the aforesaid fixed rate. Such an arrangement, particularly when combined with the noted undulatory manipulation of the material, serves to considerably improve production rates as well as afford a capability for producing materials of greater thickness and fiber loadings, ultimately deriving higher strengths. In another embodiment of the invention, four continuous belts are utilized in the fashion described, those adjacent belts positioned upstream along the production path being arranged to assert a first wet out action upon the composite and the next succeeding pair of belts asserting another wet out action specific to the altered state of the composite as it passes through the first pair of belts. Additionally, the operator is afforded the opportunity to alter the degree of compression exerted by any given pair of continuous belts. A highly desirable flexibility thus is afforded the operator with the four belt system. By utilizing a steel mesh belt of relatively coarse mesh configuration, larger composite sizes can be treated initially without causing the resin component to migrate away from the fibers or "squeeze out". In effect, the resin is trapped in a myriad of pockets to inhibit such migration at the upstream stage of treatment. The downstream pair of belts then can be provided having a finer mesh configuration to effect the final and thorough wet out of all fibers within the larger composite structure. As a consequence, composite structures exhibiting considerably enhanced strength and size can be produced where such production was heretofore impossible at practical and economical production rates.